Sedimentology, Stratigraphy, and Crustal Evolution of the Archean Greenstone Belt near Sioux Lookout, Ontario

In the Archean greenstone belt at Sioux Lookout, N.W. Ontario, a lowermost belt of volcanic rocks is unconformably overlain by sediments of the Abram Group. Our mapping has subdivided the Abram Group into three formations. The lowest (Ament Bay Formation) consists of interbedded conglomerates and sandstones. The sandstones contain large-scale cross-stratification, and the conglomerates and sandstones both lack graded bedding. The depositional environment was a subaerial alluvial fan—this is the first description of such a feature based upon modern sedimentological work in Archean rocks. The Daredevil Formation conformably overlies the Ament Bay Formation, and is composed of felsic and basic tuffs, and some interbedded turbidites. The uppermost (Little Vermilion) Formation is composed entirely of turbidites.The petrography of the sand, and large clasts, in the Ament Bay Formation indicates derivation from a dominantly granodioritic terrain. Some granodiorite boulders contain greenstone xenoliths, implying intrusion of the granodiorite after formation of the lowermost belt of volcanic rocks.In a final section of the paper, we define two Archean facies associations—a Resedimented association containing turbidites, pebbly mudstones, resedimented conglomerates, and basinal black argillites; and a Continental association, containing alluvial fan deposits, and possible coastal deposits from South Africa and Australia. The facies sequence in the Sioux Lookout belt is Continental followed by Resedimented facies, the reverse of the normal geosynclinal flysch (resedimented) → molasse (continental) sequence.